1. Field of the Invention
The present invention relates to an exposure apparatus having a laser light source or a working apparatus such as a repairing apparatus for a wafer or a mask by using a laser beam, and more particularly to an exposure apparatus for exposing a pattern formed on a mask to a photo-sensitive substrate such as a semiconductor wafer through a projection optical system in a lithography process in the manufacture of a semiconductor integrated circuit.
2. Related Background Art
In the lithography process in the manufacture of the semiconductor integrated circuit, an exposure apparatus (stepper) for exposing a pattern formed on a mask to a member to be exposed (resist) applied on a surface of a semiconductor wafer through a projection optical system has been known.
Recently, as the integration density of the semiconductor integrated circuit increases, a minimum pattern of the circuit tends to be further reduced. As a result, a control precision of an exposure amount for the stepper requires the reproducibility of in the order of .+-.1%.
An exposure light source which uses an excimer laser which can attain a higher resolution power instead of a mercury lamp which was heretofore widely used has been developed.
Usually, a pulse light source such as an excimer laser has an energy amount which varies every pulse. In this case, the pulse energy varies from several % to ten plus several % with respect to its target value. Thus, the control method for achieving the desired exposure control precision described above includes the methods (a) and (b) described below.
(a) A pulse count control system (U.S. Pat. No. 5,097,291) in which the pulse energy of the laser is reduced or the exposure energy is reduced by a dimmer provided in the exposure apparatus and the exposure is made with a larger number of pulses to reduce an effect of the energy variation per pulse for the accumulated energy.
(b) A correction exposure system in which a coarse exposure which provides a slightly lower exposure than a target exposure is given and a remaining required exposure is given by a correction exposure by one or plurality of pulses having the exposure energy thereof lowered by a dimmer (light intensity attenuation filter) provided in the exposure apparatus.
In addition, a high voltage control system in which the pulse energy is adjusted by varying a high voltage of a laser every one or several pulses has been known. However, the above methods (a) and (b) are widely used because of the ease of the control system.
When the prior art exposure system is used, the following problems are encountered.
In the method (a) above, the number of exposure pulses is larger than that of the method (b). As a result, the time of exchange of the consuming parts such as the electrode material and the electrode element in the laser light source per wafer exposure is short and the maintenance period is shortened and a downtime of the apparatus increases. Further, the number of times of exchange of gas in the laser light source increases and the running expense increases.
On the other hand, in the method (b), the laser light emission is temporarily stopped after the completion of the coarse exposure during the exposure of one chip, and the dimmer is activated and then the laser light emission is resumed to conduct the correction exposure. Accordingly, the throughput is lower than that of the method (a) unless the positioning time of the dimmer is sufficiently short.
For example, in order to prevent the throughput from being reduced when 60 exposure pulses required in the method (a) is reduced to 25 pulses in the method (b), the time relation of the exposure process of one chip is represented as follows. EQU 60/500.gtoreq.25/500+t
where 500: laser oscillation frequency (approximately 500 Hz for a currently widely used laser)
t: activation setting time of the dimmer
From the above relation, the setting time t of the dimmer is given by t.ltoreq.70 msec. Thus, the high speed activation of 70 msec or less is required for the activation time of the dimmer and this poses a mechanical and control restriction.
The light emission frequency of the laser will tend to become higher in the future and the restriction will become more severe. For example, the dimmer may be continuously rotated by a motor to be moved into and out of a light path of a laser beam and the laser beam may be emitted when the dimmer coincides with the laser light path. In this method, however, it is difficult to activate the laser at a maximum frequency in order to absorb variation in the rotation speed and the motor failure causes the increase of the downtime of the exposure apparatus.
In both the methods (a) and (b), the dimmer may be switched before the start of the exposure in order to attain the minimum number of exposure pulses to meet the exposure control precision depending on the target exposure, and such activation time is added to the exposure processing time.